In recent years, there has appeared another group of nematic liquid crystal displays, that is, a group of surface-breaking nematic displays (for example, International Paten WO97/17632). Those displays have bistability, and the orientation thereof remains indefinitely without consuming energy. Energy supply is necessary only to switch between two states. With use of such features, those displays are increasingly employed for applications that are usable for a long period of time without a power supply cable (such as electronic books and electronic shelf labels).
Manufacturing of displays of this type has difficulty mainly in the point that the surface anchoring needs to be broken. That is, reproducible low energy anchoring is necessary. The bistable nematic displays which use weak anchoring are manufactured by the following method as a typical example. That is, liquid crystal is arranged between two glass plates coated with a conductive layer made of indium tin oxide (ITO). One electrode includes a coating for obtaining high pretilt angle or strong azimuthal anchoring, and the other electrode includes a coating for obtaining low pretilt angle and weak azimuthal anchoring energy. Further, two polarizers are arranged on each side of the cell with appropriate orientations.
The principle of this “bistable” technology lies in the presence of two stable states, that is, a uniform state and a 180-degree twisted state, without, any need to apply an electric field. In the following description, the uniform state is referred to as a U-state (Uniform), and the 180-degree twisted state is referred to as a T-state (Twist). The two states of U and T correspond to minimum energy levels. The two states of U and T are balanced an the case of using nematic liquid crystal having positive dielectric anistropy, for example, pentylcyanobiphenyl (known as “5CE”) doped with a chiral additive. This type of device is generally called a Binem device.
This display technology utilizes the fact that, by applying an electric field of a specific shape and intensity to pass from one state to the other state, it is possible to break the weak anchoring while maintaining the strong anchoring state as it is. When the electric field is applied perpendicularly to the cell, similarly to the “black” state in the TN technology, a homeotropic texture is induced, in which molecules close to the low anchoring energy surface are perpendicular to the low anchoring energy surface. This non-equilibrium texture is the transition state, which can be switched to any one of the two stable states. When the application of the electric field is stopped, the state changes to the one or the other stable state depending on whether the elastic coupling effect or the hydrodynamic coupling effect is stronger.
In order to facilitate the cell switching between the T-state and the U-state, the weak anchoring may have a small tilt angle (<1°). On the ether hand, the strong anchoring may be obtained by an alignment film which is generally used for TN liquid crystal or STN liquid crystal having a certain pretilt angle. When the strong anchoring is tilted in one direction and the weak anchoring is tilted in the same direction, a transition electric field induced anchoring breakage, which is called a first-order breakage, allows switching from the T-state to the U-state. Switching free, the U-state to the T-state is achieved by the above-mentioned hydrodynamic coupling.
Various means have already been proposed for anchoring liquid crystal materials. However, although there are means that can generate sufficiently strong anchoring, there are only very few means teat can ensure weak anchoring.
As means that can ensure weak anchoring, the following method of manufacturing a liquid crystal cell is proposed in Patent Literature 1.
The weak anchoring is achieved by the method involving the steps of: attaching, to a substrate, a polymer, copolymer, or terpolymer selected from polymers and copolymers each formed of a poly(vinyl chloride-co-vinyl alkyl ether) type polymer or copolymer, or a poly(vinyl chloride-co-vinyl aryl ether) type polymer or copolymer; stabilizing the coating of the above-mentioned polymer; and causing azimuthal alignment of the coating to induce controlled azimuthal anchoring of the liquid crystal.
According to Patent Literature 1, the coating is stabilized thermally and/or by exposure to ultraviolet, and the pretilt angle of the nematic liquid crystal is small (0°<Ψ<1°, preferably 0.1°<Ψ<0.5°). As a result, it is possible to form a low energy anchoring layer in the bistable nematic liquid crystal cell.